Add note about when to use HGlobal allocator APIs.

[AaronRobinsonMSFT]

Fixes #10398

[learn-build-service-prod]

Learn Build status updates of commit cb7e549:

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[learn-build-service-prod]

Learn Build status updates of commit 2690f8c:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Runtime.InteropServices/Marshal.xml	✅Succeeded	View

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[learn-build-service-prod]

Learn Build status updates of commit e3de014:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Runtime.InteropServices/Marshal.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

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[AaronRobinsonMSFT]

@gewarren Can you take one more look at my updated phrasing? Thanks.

[gewarren]

LGTM